1. Field of the Invention
The invention is directed to a ball bearing having a first and a second race and, disposed in a gap between the two races, at least one row of balls of radius RK, that roll along facing tracks of the two races, wherein centers of the balls of a row move on a circular path that is surrounded by a torus circumscribing all the balls of the row and having the toroidal radius RK, a toroidal angle coordinate φ and a poloidal angle coordinate θ, and wherein each track has with each ball two nearly punctiform contact areas or contact points P1, P2; P3 P4 at the respective contact angle θP1, θP2; θP3 and θN, and wherein the cross sections of the tracks in the region of the contact angles θP1, θP2; θP3 and θP4 have transverse curvatures possessing finite radii of curvature RL1 . . . RL4 each of which is greater than the ball radius RK: RLv>RK.
2. Description of the Prior Art
Such four-point bearings exist in various forms, for example as single- or multi-row ball bearings. The tracks used for such bearings often have a gothic profile, i.e., the transverse curvature sectionally follows a circular path segment, but two such circular path segments join at approximately the middle of the path to form an acute angle in the manner of a gothic arch. In this way, it is possible to have two contact points between a track and a ball despite radii of curvature RL1 . . . RL4 that are consistently greater than the ball radius RK: RLv>RK.
The poloidal angle assumed by a contact point, or contact area, is commonly referred to as the contact angle. Contact angles are often approximately between 40° and 50°, particularly approximately 45°, or between −40° and −50°, particularly approximately −45°, relative to a plane passing through centers of the balls of a row.
Due to this preferred contact-angle position, load components in the axial and radial directions always occur during load transfer. Even under exclusively axial loading, this arrangement always results in radial load components as well, which, in the presence of large bearing diameters of, for example, more than 0.5 m, preferably 1.0 m or more, particularly 2.0 m or more, cause radial expansion of the outer ring, on the one hand, and constriction of the inner ring, on the other.
Under combined loading, i.e., superimposed axial, radial and/or tilting moment loads, the races undergo elliptically shaped deformation. At the location where the highest load is being transferred, the outer race expands the most and the inner race constricts the most. Both races deform to ellipses whose principal axes are, however, rotated with respect to each other, particularly by approximately 90°, such that, for example, the large half-axes of the outer race approximately coincide with the small half-axes of the inner race, and the width of the gap between the two races consequently varies with the toroidal angle cp.
Since the two races are farthest apart in the regions where the highest load is being transferred, the contact angles are displaced the most at those locations. Depending on the deformation and rigidity of the adjacent construction, the contact angles can be shifted by up to ±65° or ±70°, or even as much as ±75°, or more.
Under high stress or some degree of bias, instead of a contact point between ball and race there is an area of contact, preferably of approximately elliptical shape, the so-called pressure ellipse. If, due to large displacement of the contact angle, this contact area, or pressure ellipse, approaches the bearing gap, it may be sheared off by the edge between the track and the bearing gap. If this occurs, not only does the loading of the ball in the rest of the contact area increase, but also, in particular, heightened edge pressures are created and will soon cause damage to the balls and tracks. The greater the diameter of the ball bearing and the lower the structural rigidity of an adjacent construction, the stronger this effect. Under unfavorable conditions, therefore, a four-point bearing must be abandoned in favor of a more elaborate and expensive bearing design, for example a multi-row roller bearing having at least one row of rollers with a contact angle of 90° for axial and tilting-moment loads, and at least one row of rollers with a contact angle of 0° for radial load transfer.